Finite impulse response (FIR) filters are commonly implemented in photonic integrated circuits (PICs) to make devices such as asymmetric Mach-Zehnder interferometers (AMZIs), array waveguide gratings (AWGs), or wavelength division multiplexing (WDM) devices. FIR stages can also be assembled in series to make a more complex filter. FIR filter devices may be deployed in environments where there can be wide variations in ambient temperature, due to both environmental changes and the use of heat-dissipating components.
FIR filters implemented in PICs have several technical problems. As the materials used to construct PICs have a temperature-dependent index of refraction, temperature changes to FIR filter devices can affect the spectral characteristics of the FIR filter. Correcting for this in a deployed PIC typically requires maintaining the device temperature within a narrow range; this maintenance may require energy-intensive heating of the entire PIC package.
Furthermore, especially in the case of silicon-on-insulator (SOI) PICs, fabricated devices commonly have deviations from the design which lead to changes in the transmission spectrum. This is because SOI waveguides have a strong refractive index contrast between the silicon core and silicon dioxide cladding, and also because of the small dimensions typical of SOI waveguides; these small dimensions lead the effective index of a mode to be strongly dependent on changes in waveguide cross sectional dimensions. Fabrication deviations may be local, affecting different regions of a PIC to different degrees, or global, causing a constant deviation of the waveguide cross section from the design. Both local and global deviations detune the spectral response of an FIR filter.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.